1. Field of the Invention
The invention concerns a conical microstrip belt antenna with good radio frequency performance that can be designed and printed on a flat substrate. It also concerns the preparation of an antenna of this kind on a flat substrate.
2. Description of the Prior Art
An electromagnetic wave, characterized among other parameters by its wavelength, conveying energy and usually embodying information can propagate in various media, the main media of interest in the present context being:
guided propagation media (cables, lines, waveguides, etc.), and PA1 unguided propagation media (free space, whether homogeneous or not, whether isotropic or not, etc.). PA1 One characteristic parameter of an electromagnetic wave is its wavelength .lambda. (the ratio of the speed of light to the frequency of the signal transmitted). PA1 the Voltage Standing Wave Ratio (VSWR), which reflects the quality of matching, i.e. the quantity of energy transmitted from the feed line to the antenna; the better the quality of matching, the closer to VSWR=1; PA1 the antenna radiation pattern which is a diagram showing the distribution in space of the vector E (electromagnetic field) of the wave, with which the standard parameters are associated (gain, directivity, efficiency, -3 dB aperture, coverage probability, etc.). PA1 either guide an electromagnetic wave (microstrip line), or radiate an electromagnetic field (microstrip antenna). PA1 either the air-substrate interface, or the air-conductor-substrate interface. PA1 the radiation pattern is largely predictable, PA1 the dimensioning of these antennas to resonate a given frequency is well understood. PA1 .lambda.o is the wavelength in air at the frequency fr (i.e. the ratio of the speed of light to this frequency), and PA1 W is the width of the patch, obtained from a simple formula given in the above work: ##EQU3## PA1 serial, PA1 parallel, PA1 a combination of the two. PA1 thin, PA1 light in weight, PA1 of low cost (being easy and quick to manufacture), PA1 that can be "conformed", for example to apply them to cylindrical or conical structures. PA1 enhanced efficiency (low losses through spurious radiation), PA1 a wider usable band of frequencies. PA1 equidistantly and conveniently spaced (by a distance less than or close to one half-wavelength), PA1 situated at the same altitude, i.e. at the same height relative to the reference base of the frustum, PA1 fed with the same amplitude and phase (to within a given tolerance). PA1 D1: U.S. Pat. No. 3,914,767, PA1 D2: U.S. Pat. No. 4,101,895, PA1 D3: U.S. Pat. No. 3,798,653, PA1 D4: U.S. Pat. No. 4,980,692, PA1 D5: U.S. Pat. No. 4,051,480, PA1 D6: U.S. Pat. No. 2,490,024, PA1 D7: U.S. Pat. No. 4,160,976, PA1 D8: U.S. Pat. No. 4,816,836, PA1 D9: EP-A-0,575,211. PA1 the N radiating patches are divided into S identical sub-arrays and the length .DELTA.La.sub.i of the rectilinear segments of stage i and the angle .gamma..sub.i between adjacent segments are such that: PA1 the radiating elements are trapezoidal in shape, PA1 the radiating elements are rectangular in shape, PA1 the radiating elements are circular in shape, PA1 the radiating elements are those described in document D9, PA1 i.e. each patch is formed of a conductive loop of constant width 1 around a dummy patch that is not energized and separated from the dummy patch by a continuous closed-loop slot of constant width l coupling the loop and the dummy patch, PA1 the length of the rectilinear segments is made at least equal to approximately one-quarter the wavelength in the dielectric material, and PA1 the height of each stage is the same. PA1 the N radiating patches are divided into S sub-arrays, PA1 each feed array is such that the line lengths between the common point and the radiating patches of the sub-array are substantially identical to within c/(F.sub.0 .sqroot..di-elect cons.e)where c is the speed of light and .di-elect cons.e is the effective dielectric constant of the propagation medium constituted by the substrate and conductive lines, PA1 the tree-structure array is formed on the same surface of the dielectric material layer as the sub-array of radiating patches, PA1 the tree-structure array is made up of n2 stages of second order dividers and n3 stages of third order dividers, in any order, PA1 the dividers are conformed within the same stage i so that each, when developed on a plane, is an integer number of substantially identical straight line segments with equal angles .gamma..sub.i between them, the dividers of a same stage approximating arcs of a common circle concentric with the circular arc constituted by the circular reference line when developed on the plane.
An antenna can be regarded as an interface between these two types of media, enabling total or partial transfer of electromagnetic energy from one to the other. The transmit antenna passes this energy from the guided propagation medium to the unguided propagation medium and the receive antenna reverses the direction of transfer of energy between the media. In the remainder of this description the transmit antenna is usually referred to by implication. However, the principle of reciprocity means that all the stated properties apply to the receive antenna.
The feed circuit(s) or device of the antenna embodies components of all or part of the guided propagation medium directing or collecting the electromagnetic energy to be transferred and embodying passive or active devices, reciprocal or otherwise.
A unit antenna is often associated with one or more geometrical points called phase centers from which the electromagnetic wave appears to emanate for a given direction of transmission in the case of a transmit antenna.
The antenna resonates at the frequency or frequencies for which the transfer of energy transmitted from the feed line into space via the antenna is optimal, which can be expressed in mathematical terms as follows: at the resonant frequency fr the complex impedance Z at the antenna input has a null imaginary part and a maximal real part.
At microwave frequencies, the locus of impedances is plotted on the Smith chart on which each resonance appears as a loop.
With current measuring instrumentation, this resonance is "seen" through the matching that is characteristic of transfer of energy from the feed line to the antenna. This view of the behavior of the antenna can be termed the response of the antenna and is quantified by mismatch loses or the Voltage Standing Wave Ratio (VSWR) defined below.
If Z is the impedance at the point at which the matching is measured and Zc is the characteristic impedance of the feed line (under the standard usually adopted, Zc=50 ohms), then the reflection coefficient is the complex ratio: EQU P=(z-1)/(z+1)
where z=Z/Zc. The VSWR is defined as follows: EQU VSWR=.vertline.(1+.vertline.P1)/(1-.vertline.P1).vertline.
Unfortunately, a radiating element does not usually have an impedance equal to Zc. A "matching" interface must be inserted between the radiating element and the cable conveying the energy. Its purpose is to convert the impedance Ze at the antenna input to an impedance presented to the feed cable which is near the impedance Zc at the operating frequencies of the antenna, with a VSWR close to 1.
The radio frequency performance of an antenna is characterized by parameters including:
By convention, the radiation pattern is shown in a system of axes centered at a point on the antenna (its phase center, if possible) and embodies a set of "cross-sections" in a standard system of spherical coordinates (.theta..phi.). A "constant .phi." section shows the curve of variation of the field E projected onto a given polarization (either E.theta. or E.phi.), for .theta. varying from 0.degree. to 180.degree. (or from -180.degree. to +180.degree. ). Similarly, a "constant .theta." section is a curve showing the variation of the field E projected onto a given polarization (either E.theta. or E.phi.), for .phi. varying from 0.degree. to 360.degree..
An association of unit antennas is called an antenna array if the unit antennas have common parts in their feed circuits or if coupling between the unit antennas makes the overall radiation pattern of the array in a given range of frequencies dependent on that of each of the unit antennas or radiating elements.
The array obtained by distributing antennas similar to one or more given unit antennas over a given surface is often called an array antenna, usually implying the concept of geometrical repetition of the unit antennas.
Array antennas are usually employed to obtain a highly directional radiation pattern in a given direction relative to the array.
The spacing .DELTA. between the phase centers of the unit antennas of the array, relative to the wavelength .lambda.o in the propagation medium, for example air, is a critical parameter.
For example, for values of .DELTA./.lambda.o&gt;0.5, significant array lobes outside the wanted radiation area penalize the energy transmission balance in the unguided propagation medium.
The microstrip technology resides in stacking a plurality of layers of conductive or dielectric material such as a dielectric substrate (glass fiber-reinforced PTFE, for example) coated on its lower side (or I side) with a conductive film (of copper, gold, etc.) and carrying on its upper side (or S side) a conductive film cut into a given geometrical design (usually referred to as "patches").
This system can:
The current propagation medium is:
In the former case the effective dielectric constant of the medium is defined by convention as: EQU .di-elect cons.e=(.di-elect cons.r+1)/2
Where .di-elect cons.r is the dielectric constant of the substrate. In the later case: ##EQU1## where h is the thickness of the substrate and W is the width of the conductive strip.
Various types of component and other devices (possibly active components and devices) can usually be mounted on the S side of the structure.
By definition, a microstrip antenna is a geometrically shaped conductive material element on the S side of a dielectric layer.
A rectangular or circular shape is often chosen, for the following reasons:
A rectangular microstrip patch is to some degree equivalent to two parallel slots coinciding with two radiating edges of the rectangle. The selection of the edges of a rectangular patch which must radiate (and by extension those which must not radiate) is effected by an appropriate choice of the area of the rectangle connected to the feed circuit.
The rectangular patch is usually fed near or at the median line joining the sides which are to radiate. In this way, the mode excited in the resonator produces linear polarization of good quality. The direction of this polarization is perpendicular to the radiating edge of the patch.
This connection can be made through the dielectric substrate or at the periphery of the patch by means of a microstrip line on the S side (this is sometimes called "coplanar feeding"); see for example French Patent 2,226,760.
It is essentially the distance L between these edges (called the "length" of the patch) which determines the resonant frequency of the antenna.
Equations and charts have been developed for this.
For example, according to "MICROSTRIP ANTENNAS", I. J. Bahl and P. Bhartia, ARTECH HOUSE, 1980, to resonate at the frequency fr a rectangular patch must have a length L such that: ##EQU2## .di-elect cons.r is the dielectric constant of the dielectric substrate, h is the height (or thickness) of the substrate,
The width W of the patch conditions the radiation pattern of the antenna.
The width W chosen conditions to a large degree the quality of radiation, i.e. its efficiency and its shape.
According to the above document, the radius of a circular patch is given by the following formula: ##EQU4##
Any microstrip patch can be used as an element of an array of the following types:
This technology can provide antennas (or antenna arrays) which are:
The microstrip antenna is in fact an electronic resonator which by construction has a high Q. Because of this, antennas using this technology always have a narrow bandwidth, i.e. resonance occurs only at the frequency for which the antenna is dimensioned and at frequencies very close to this frequency.
As already mentioned, a matching interface (or feed system or array) is usually required between the radiating patch and the feed cable. The simplest solution is usually to print the matching interface on the same side of the substrate as the radiating patch itself. The matching interface most commonly used, because of its simplicity, is the so-called "quarter-wave" matching interface. Its performance is mediocre, however. In the microstrip technology, the impedance of a line of width W printed onto a substrate of thickness e with dielectric constant .di-elect cons.r is given by the following equation (see "Computer-Aided Design of Microwave Circuits", K. C. GUPTA, RAMESH GARG AND RAKESH CHADHA, Artech): ##EQU5## for W/e greater than or equal to 1, or Z=60ln (8e/W+0.25W/e) for W/e less than or equal to 1.
This equation indicates that on a given substrate the characteristic impedance of a microstrip line is conditioned by the width of the line. The wider the line the lower the impedance.
Let Ze denote the impedance at the entry point of the radiating patch. If Zd is the impedance required at the interface with the feed system (the cable, for example), the quarter-wave matching interface is then a section of printed line whose length is .lambda.g/4 (where .lambda.g=.lambda./.di-elect cons.e is the wavelength in the dielectric) and has a characteristic impedance Zc=.sqroot.(Ze/Zd).
There are other types of matching interface (the "streamlined" line, for example), whose complexity often goes hand in hand with:
A number of applications of the microstrip technology to so-called "conformed" antennas, i.e. antennas applied to a non-plane surface, have already been described.
For example, in French patent application 92-07274 the patches are distributed over the surface of a cylinder. The objective of this antenna, called a "belt antenna" is to produce an omnidirectional radiation patch, i.e. to provide a gain which is as uniform as possible in all regions of space. The patches are equidistant and can be grouped into identical sub-arrays also incorporating the feed array for routing the signal to each element. All are fed with the same amplitude and the same phase (to within a given tolerance) to guarantee a regular radiation pattern.
The invention is also directed to achieving good radio frequency performance (in particular with regard to the radiation patch), but from a microstrip belt antenna applied to a conical body, following preparation on a plane substrate according to a realistically and reliably determined design, the law for feeding of the antenna of the present invention being identical to that of the preceding example, for example.
The only problem in designing a cylindrical belt antenna like that described in French patent application 92-07274 is that of designing a one-dimensional feed array (a single row of elements to be fed) which is correctly matched (VSWR of approximately one) at the operating frequency or frequencies. This does not present any great problem to the person skilled in the art, using either standard equations or preferably a CAD system. Each sub-array is designed on a plane surface and retains its matching properties when wrapped onto the cylindrical body.
Designing this type of antenna for application to a conical body is more complex, if all the radiating elements must be:
The above are the necessary and sufficient conditions for obtaining an omnidirectional radiation pattern.
The present invention proposes a method of designing and fabricating this type of antenna on a plane surface like a printed circuit before applying it to a cone and a type of antenna that can be fitted onto any given cone with the only modification of the cone that is required being the provision of one or more holes for the feed cable(s).
The following patents (identified as documents D1-D9) discuss or touch on this problem:
These prior art references describe concepts based on slot technology (documents D3 and D6) or microstrip technology or techniques derived therefrom.
Documents D3 and D6 describe antennas which are an integral part of the structure on which they are disposed. This does not correspond to the requirements stated above (minimal impact on the support structure).
Documents D1, D2, D4 and D5 have frequent recourse to numerous and costly short-circuits through the substrate (in order to guarantee sufficient bandwidth--which the previously mentioned French patent application 92 07274 can avoid) and provide no information as to the feed system or array of the antenna, so that it may be assumed that a technique other than the microstrip technique is used. One of the major benefits of this technique is precisely the fact that it enables combination on the same support (the dielectric substrate) of the feed array and the radiating elements, so eliminating many of the mechanical constraints encountered with antennas using other technologies.
Document D7 proposes a microstrip antenna applied to a cylindrical body with no precise information as to the design or dimensions of the feed array or system. Document D8 concerns a two-layer array antenna structure on a cylindrical or conical surface but gives no specific information as to the radiating patches or their feed array.
Document D9, already cited, concerns only a cylindrical belt antenna. However, the unit radiating patch described in this document combines a thin dielectric substrate with a wide bandwidth. This patch can be used with advantage in the present invention.